1. Field of the Invention
The present invention relates to semiconductor lasers, and more particularly to a low-noise, self-pulsation type semiconductor laser suitable as a light source for recording and reproducing information on an optical disk or the like.
2. Description of the Background Art
In the case that a semiconductor laser for lasing in a uniaxial mode receives laser light feedback reflected from an optical disk surface, the lasing state of the semiconductor laser changes unstably due to the interference with the optical feedback, thereby causing noise. Such noise is called optical feedback noise, which becomes a major obstacle to using the semiconductor laser as a light source of an optical pick-up for reproducing information on an optical disk or the like.
Conventionally, in order to reduce the optical feedback noise, a high frequency voltage has been superposed on a driving voltage of the semiconductor laser to lower the coherence of the laser light. However, this method requires an external circuit for the superposition of e high frequency voltage, so that costs and sizes of optical pick-up parts are increased and further the circuit is liable to emit undesirable electromagnetic waves.
In a self-pulsation type semiconductor laser provided with a saturable absorption region in an optical waveguide, on the other hand, the saturable absorption region functions to cause self-pulsation of the lasing intensity within a frequency range from some hundred megahertz to some gigahertz, so that the coherence of the laser light can be lowered. Further, the superposition of high frequency voltage is unnecessary in the self-pulsation type semiconductor laser and thus an external circuit therefor can be eliminated. As such, it becomes possible to produce compact pick-up parts not emitting electromagnetic waves.
FIG. 11 shows a self-pulsation type semiconductor laser of a first conventional example in a schematic cross section. Here, an AlGaInP-based semiconductor laser is shown as an example of the self-pulsation type semiconductor laser for emitting light of red color.
The semiconductor laser of FIG. 11 is provided with an n type GaAs substrate 1 and a semiconductor stacked-layered structure epitaxially grown thereon. Specifically, the semiconductor stacked-layered structure has an n type AlGaInP first clad layer 2, a GaInP active layer 3 and a p type AlGaInP second clad layer 4 which are successively stacked on the substrate. Second clad layer 4 has a striped ridge portion, with both sides of the ridge portion (i.e., non-ridge portions) being thinner than the ridge portion. A p type GaInP intermediate layer 5 and a p type GaAs contact layer 6 are formed on the ridge portion of second clad layer 4. An n type GaAs embedding layer 9 is formed on either side of the striped ridge portion. Light confinement in a horizontal direction is achieved by an effective refractive index difference Δn generated between the ridge portion and the non-ridge portion. A p-side electrode 10 is provided on the upper surface of the semiconductor stacked-layered structure, and an n-side electrode 11 is provided on the backside of substrate 1.
In the self-pulsation type semiconductor laser of the first conventional example, the light confinement in the horizontal direction parallel to active layer 3 is weaken by designing effective refractive index difference Δn to be small, so that the light intensity within the active layer is increased in the side regions corresponding to both sides of the stripe. The side regions can serve as the saturable absorption regions to realize the self-pulsation.
In the structure of the first conventional example, which is commonly used in a self-pulsation type semiconductor laser, the regions of the active layer corresponding to the both sides of the stripe are used as the saturable absorption regions. Thus, it is necessary to increase the light intensity in the regions, and the light confinement in a direction perpendicular to the active layer also needs to be set high. As a result, the divergent angle of the emitted light increases in the direction perpendicular to the active layer and thus the ellipticity of the beam cross section also increases. Further, the light intensity at the end face of the active layer increases, thereby lowering the COD (catastrophic optical damage) level.
To solve the foregoing problems, the present inventors have provided a self-pulsation type semiconductor laser in which it is unnecessary to increase the light confinement in the stacked-layered direction perpendicular to the active layer and then the ellipticity of the beam cross section is small, by incorporating a saturable absorption region in the embedding layer of the first conventional example (see Japanese Patent Laying-Open No. 9-181389). This self-pulsation type semiconductor laser is shown as a second conventional example in FIG. 12 in a schematic cross section.
The semiconductor laser shown in FIG. 12 is provided with an n type GaAs substrate 1, and a semiconductor stacked-layered structure grown thereon. The semiconductor stacked-layered structure includes an n type (Al0.65Ga0.35) InP first clad layer 2, a GaInP active layer 3 and a p type (Al0.65Ga0.35) InP second clad layer 4, which are stacked successively on the substrate. Second clad layer 4 has a striped ridge portion 4a, and regions (non-ridge portions) 4b on both sides of the ridge portion are thinner than the ridge portion. A p type GaInP intermediate layer 5 and a p type GaAs contact layer 6 are formed on the ridge portion of second clad layer 4. An n type Al0.6Ga0.4As layer 8d, an n type GaAs saturable absorption region 7, an n type Al0.6Ga0.4As layer 8d, and an n type GaAs embedding layer 9 are formed on either side of the striped ridge portion. A p-side electrode 10 is provided on the upper surface of the semiconductor stacked-layered structure, and an n-side electrode 11 is provided on the backside of substrate 1.
In the semiconductor laser of the second conventional example, carriers generated by absorption of laser light are accumulated in the saturable absorption region formed in the embedding layer thereby causing saturation of light absorption, so that the self-pulsation is achieved similarly as in the first conventional example. In the first conventional example, the active layer regions on both sides of the ridge can serve as the saturable absorption layers. However, electric current spreads in a lateral direction from the ridge, so that the carriers are introduced also into the active layer regions on both sides of the ridge. Thus, the change in absorbable light amount attributable to carriers generated by absorption of laser light is small. In the second conventional example, on the other hand, the saturable absorption layer is formed in the embedding layer where the current does not flow. Thus, the carriers do not exist in the saturable absorption layer during the state in absence of lasing. Accordingly, the absorbable light amount greatly changes depending on carriers generated by absorption of laser light. Further, in the second conventional example, since the excited carriers are generated in regions far away from the restart planes of crystal growth after formation of the ridge, trapping of the carriers is unlikely to occur at non-radiative centers in the vicinity of the restart planes of crystal growth, and thus the saturable absorption layer can function effectively.
In the second conventional example, although the light intensity in the saturable absorption region needs to be increased to cause self-pulsation, it is unnecessary to thicken the active layer for that purpose. Thus, it is possible to provide a self-pulsation type semiconductor laser having a small ellipticity of the beam cross section.
However, with the above-described self-pulsation type semiconductor laser having the saturable absorption region formed in the embedding layer, the self-pulsation stops at a high temperature of more than 70° C. or with an optical output of more than 6 mW.